This invention relates generally to snap acting thermostatic disc members which are used for performing control functions and more particularly to thermally responsive snap acting members embodying iron alloy materials selected for strength and temperature resistance properties for use with electrical switches employing high temperature snap acting members.
A known type of thermally responsive control member used for performing control functions has a dished shaped configuration of precisely predetermined shape which cooperates with the characteristics of the metal materials embodied in the member so that the member moves to an inverted or oppositely dished shaped configuration with snap action on the occurrence of the member reaching a selected actuating temperature. The member is then movable to return to its original dished shaped configuration on the occurrence of the member reaching a second de-actuation, or differential, temperature to reset the member. Usually there is a substantial difference between the actuating temperature and the reset temperature in order to provide hysteresis and avoid unnecessary cycling of the member.
Such thermally responsive dished shaped metal control members are formed into the precisely predetermined configurations in well known ways and are typically formed of iron alloys to provide strength where the elements are intended to operate at elevated temperatures. One known material system is the use of an iron alloy referred to herein as Alloy 10 as a low expansion layer bonded to another iron alloy referred to herein as Alloy B as a high expansion layer which can provide relatively stable snap acting disc members at low actuation temperatures. However, when the disc members are formed so that the actuating temperature is increased the disc members become unstable above some threshold level dependent upon the geometry of the member and the sigma distribution increases so that the disc members start to fall out of a tolerance range for a given application. Such known disc members having a moderately high actuation temperature, e.g., 130.degree.-150.degree. C., frequently are unstable undergoing significant variations or changes in thermal response characteristics during snap action movement and thermal cycling of the members so that even if they originally met desired performance specifications, over time many drift out of the tolerance range. For example, for certain geometries for some members having an actuating temperature as low as approximately 130.degree. C. the temperature response of the members upon snapping are immediately found to display as much as 1.degree. to 5.degree. C. decrease in actuating temperature response characteristics and undergo additional drift in response characteristics of similar or greater magnitude over a typical service life. For any given geometry, the above problems are exacerbated as the actuation temperature is increased.
It is an object of the present invention to provide thermostatic snap acting disc members which have a high actuation temperature, for example, on the order of 150.degree. C. and higher. Another object is the provision of such high temperature thermostatic snap acting discs which are stable having a selected actuating temperature which essentially does not change over time and which have a low sigma distribution relative to the tolerance range of a given application. Yet another object is the provision of high actuating temperature thermostatic, snap acting discs, for example, in the order of 150.degree. C. or higher, which have a wide differential de-actuating temperature of up to 200.degree. C. or more below the actuation temperature making them useful as non-resettable control members. Still another object of the invention is the provision of temperature responsive switches employing such high temperature snap acting thermostatic discs.